alt Hacker NewsiOS asks you if you want to allow notifications when each new app is started. You can just say no there and you're done.
It would be better if they were totally opt-in of course (1), but that's not bloody likely to happen.
(1) As in off by default with no questions.
I can see a certain category of people screaming that WhatsApp calls are broken if that were to pass… but I do agree that no one would scream louder than app makers wanting to retain their share of human brain attention.